Just another small answer to a possible annoying issue if you don’t know what to do. In some circumstances you might change you Gnome panel configuration file or an update might change it or the file can get corrupt or maybe from different other reasons you might want to re-set your Gnome panel.
How can you do that?
Well in Ubuntu you just need to run these commands:
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gconftool-2 --shutdown rm -rf ~/.gconf/apps/panel pkill gnome-panel